An 83-year-old retired priest was amongst 29 people arrested during a demonstration against the proscription of Palestine Action.
Reverend Sue Parfitt was led away by officers on Saturday for supporting the newly banned protest group whilst sitting in her camp chair.
She branded the ban "total nonsense" and said it symbolised a "loss of civil liberties in this country".
The Metropolitan Police commissioner defended the detention of the elderly protester.
An 83-year-old retired priest was amongst 29 people arrested during a demonstration against the proscription of Palestine Action
PA
After the arrest, Sir Mark Rowley said: "The law does not have an age limit."
The pensioner, from Henbury in Bristol, was detained in Parliament Square on the same day the group became a proscribed organisation under the Terrorism Act 2000.
The Met Police commissioner said: "It is not about protest. This is about an organisation committing serious criminality."
Officers encircled dozens of demonstrators gathered beneath the Mahatma Gandhi statue in Parliament Square, where protesters held placards declaring: "I oppose genocide. I support Palestine Action".
Reverend Sue Parfitt was led away by officers
PA
The Metropolitan Police confirmed that arrests were being made, they said: "The group is now proscribed and expressing support for them is a criminal offence."
All 29 arrested individuals have been released on police bail whilst investigations continue.
The demonstration occurred after MPs voted overwhelmingly to approve Home Secretary Yvette Cooper's decision to proscribe Palestine Action as a terrorist organisation.
The law change places the group in the same category as al Qaeda, ISIS and Hezbollah, with support for the organisation now punishable by up to 14 years imprisonment.
The Metropolitan Police commissioner defended the detention of the elderly protester.
PA
The decision to ban Palestine Action followed an incident at RAF Brize Norton in Oxfordshire on 20 June, which the group claimed credit for.
Two Voyager aircraft were sprayed with red paint, causing approximately £7million worth of damage, according to police.
Cooper had previously described the group as "disgraceful" and highlighted their "long history of unacceptable criminal damage".
In a letter to the Home Secretary, protesters declared: "We do not wish to go to prison or to be branded with a terrorism conviction, but we refuse to be cowed into silence by your order."